One method to control intake and exhaust valve operation during engine operation is described in U.S. Pat. No. 6,138,636. This method provides hydraulically operated valve switching means on an intake side and on an exhaust side, respectively, for switching intake valves and exhaust valves of the partial cylinders between a driving state and a drive-free state, wherein the intake valves and the exhaust valves of the partial cylinders are switched, during the partial-cylinder operation.
The inventors herein have determined that the above-mentioned approach can have several disadvantages. First, the approach simply focuses on deactivating intake and exhaust valves to deactivate cylinders. The approach determines the state of cylinder deactivation based on engine load, a prescribed engine speed range, engine coolant temperature, and when the vehicle speed is above a minimum. This approach may present drivability issues when transmission oil temperature is low. For example, internal combustion engines and transmissions are designed to operate over a range of temperatures. However, during low temperature operation a temperature difference may develop between engine temperature and transmission oil temperature. When an engine is started at a low temperature and allowed to idle the engine temperature may increase, but the transmission oil temperature may increase at a different rate. If transmission oil temperature is low, and a cylinder or valve mode change occurs, a torque disturbance may be felt through the driveline by the driver.
Second, the approach may allow an engine to operate at combustion frequencies that may excite modal frequencies of a vehicle. By way of an example, if a chassis of a vehicle is excited by frequencies below 20 Hertz, the before-mentioned method may allow the chassis modal frequencies to be excited by certain cylinder or valve deactivation modes.